The present invention relates to a radiographical image reading apparatus, and more particularly, to an image unevenness compensation technique in an apparatus for reading radiographical image information accumulated and recorded in a stimulation phosphor.
Radiographical images such as X-ray images are widely used for diagnoses for disease, and for these X-ray images, there has widely been used a so-called radiograph which is obtained by irradiating X-rays transmitted through a photographic object on a fluorescent substance layer (a fluorescent screen), thereby generating visible light, illuminating this visible light on a film employing silver salt like an ordinary photographic film, and by developing the film.
Recently, however, there has been proposed a method for reading images directly from the fluorescent substance layer without using a film coated with silver salt.
As the aforementioned method, there is a method available wherein radioactive rays transmitted through an object are caused to be absorbed in a stimulation phosphor and then the stimulation phosphor is excited by light or heat energy, and thereby, radioactive ray energy (radioactive ray image information) stored in the stimulation phosphor by means of the aforementioned absorption is caused to conduct stimulation irradiation as a fluorescence, and this stimulation irradiation light is converted photoelectrically to obtain image signals.
To be concrete, U.S. Pat. No. 3,859,527 and Japanese Patent Publication Open to Public Inspection No. 55-12144 (hereinafter referred to as Japanese Patent O.P.I. Publication) disclose a method for converting radiographical images wherein a stimulation phosphor is used and visible light or infrared ray is used as a stimulation exciting light for the stimulation phosphor. This method employs a panel for converting radiographical images which are formed by providing a stimulation phosphor layer (a stimulation layer) on a support. The stimulation phosphor layer of the converting panel is irradiated by radioactive rays transmitted through an object, thus, radioactive ray energy each corresponding to the transmission rate of radioactive rays for each portion of the object is stored in the converting panel to form a latent image therein. After that, the stimulation layer is scanned by the stimulation exciting light to cause radioactive ray energy stored in the converting panel to be emitted and converted to light, thus, the light signals are converted photoelectrically to obtain radiographical image signals.
Radiographical image signals thus obtained are usually output, as they are or after being subjected to image processing, to a silver salt photographic film or a CRT to be visualized thereon, or they are digitalized for computerized image processing.
Digitalized radiographical image signals are stored in an image storage memory such as a semiconductor storage device, a magnetic storage device, an optical disk memory, or a photomagnetic storage device. After that, they are sometimes taken out to be output to a printer or a CRT to be visualized thereon.
In an apparatus for reading radiographical image information stored and recorded in a stimulation phosphor as stated above, a plurality of radiographical image converting panels are set selectively in many cases, and radiographical images stored in each radiographical image converting panel are read. In this case, occurrence of uneven and nonuniform coating on the stimulation phosphor layer of the aforementioned radiographical image converting panel is unavoidable in the course of production thereof. Therefore, it is sometimes impossible, due to the aforementioned sensitivity unevenness, to reproduce accurately the images, which deteriorates the quality of a radiographical image and its quantitativeness.
Namely, when the radiographical image converting panel has thereon a sensitivity unevenness, a shadow which is not related to an amount of radioactive rays transmitted through an object appears on a reproduced image, resulting in deterioration of reproducibility (efficiency of diagnosis in medical field) of radiographical images. In case of plural radiographical image converting panels to be set selectively on a radiographical image reading apparatus, each of them has a different characteristic of sensitivity unevenness. Therefore, it has been difficult to reproduce radiographical images accurately corresponding to different plural patterns of sensitivity unevenness.
In addition to sensitivity unevenness of each of the aforementioned radiographical image converting panels, there also is X-ray uneven generation (uneven X-ray generation) caused by a heel-effect in an X-ray generator. Therefore, when radiographical images are stored and recorded on radiographical image converting panels using plural X-ray generators, and when the radiographical images each being photographed through each panel are read by one radiographical image reading apparatus, the radiographical image reading apparatus can not recognize X-ray uneven generation because of different pattern of X-ray uneven generation of each X-ray generator. In this case again, reproducibility of radiographical images has been deteriorated by X-ray uneven generation in the X-ray generator.
When such unevenness exists, correlation between an amount of X-rays and signal value is deviated and quantitativeness of a signal value to the transmitted amount of X-rays disappears, resulting in inconvenience in measurement of an amount of destroyed bones.
The invention has been devised in view of the aforementioned problems, and its object is to provide a radiographical image reading apparatus wherein even when there are sensitivity unevenness of radiographical image converting panel and X-ray uneven generation in an X-ray generator, the reproducibility of radiographical images is not deteriorated by such unevenness.